Backward Incompatible Changes

Although most existing PHP 5 code should work without changes, please take
note of some backward incompatible changes:

The newer internal parameter parsing API has been applied across all the
extensions bundled with PHP 5.3.x. This parameter parsing API causes
functions to return NULL when passed incompatible parameters. There are
some exceptions to this rule, such as the get_class()
function, which will continue to return FALSE on error.

The behaviour of functions with by-reference parameters called by value has
changed. Where previously the function would accept the by-value argument,
a fatal error is now emitted. Any previous code passing constants or
literals to functions expecting references, will need altering to assign
the value to a variable before calling the function.

The new mysqlnd library necessitates the use of MySQL 4.1's newer 41-byte
password format. Continued use of the old 16-byte passwords will cause
mysql_connect() and similar functions to emit the error,
"mysqlnd cannot connect to MySQL 4.1+ using old
authentication."

The new mysqlnd library does not read mysql configuration files
(my.cnf/my.ini), as the older libmysqlclient library does. If your code relies on
settings in the configuration file, you can load it explicitly with the
mysqli_options() function. Note that this means the
PDO specific constants PDO::MYSQL_ATTR_READ_DEFAULT_FILE
and PDO::MYSQL_ATTR_READ_DEFAULT_GROUP are not defined
if MySQL support in PDO is compiled with mysqlnd.

The trailing / has been removed from the SplFileInfo
class and other related directory classes.

An emulation layer for the MHASH extension to wrap around the Hash extension
have been added. However not all the algorithms are covered, notable the
s2k hashing algorithm. This means that s2k hashing is no longer available
as of PHP 5.3.0.

The following keywords are now reserved and may not be used in function,
class, etc. names.